1. Field of the Invention
The present invention relates to a high-frequency amplifier and a high-frequency multistage amplifier which are used for the satellite communication, the ground microwave communication and/or the mobile communication.
2. Description of Related Art
FIG. 16 is a view showing the configuration of a conventional high-frequency amplifier disclosed in a letter xe2x80x9cUltra-Low Noise LNA with Series Drain Resistersxe2x80x9d, the Institute of Electronics, Information and Communication Engineers, Society Meeting of 1999, C-2-38, pp. 67.
In FIG. 16, 1 indicates an input terminal for receiving an input signal. 2 indicates an output terminal. 3 indicates an amplifier for amplifying the input signal. 4 indicates an input matching circuit for applying an input-side bias to the input signal. 5 indicates an output matching circuit. 6 indicates a series resistor. 7 indicates an inductor or a line for a bias feed. 8 indicates a condenser. 9 indicates an output-side bias circuit for applying an output-side bias to the signal amplified in the amplifier 3. The output-side bias circuit 9 is composed of the inductor 7 and the condenser 8.
Next, an operation will be described below.
A signal input to the input terminal 1 is received in the amplifier 3 through the input matching circuit 4. Thereafter, the input signal is amplified in the amplifier 3 while stabilizing the amplified signal in the series resistor 6. Thereafter, the amplified signal is output from the output terminal 2 through the output matching circuit 5 as an output signal. An input-side bias is applied to the input signal in an input-side bias circuit arranged in the input matching circuit 4. An output-side bias is applied to the amplified signal from the output-side bias circuit 9. Therefore, the input signal can be amplified without an unstable operation such as oscillation. Also, because the series resistor 6 for stabilization is inserted into the conventional high-frequency amplifier on the output side of the amplifier 3, a low noise characteristic can be obtained in the conventional high-frequency amplifier.
However, because the series resistor 6 for stabilization is inserted on the output side of the amplifier 3, the loss of an electric power of the amplified signal is generated in the series resistor 6 placed on the output side of the amplifier 3, and an output electric power characteristic and a third intercept point (IP3) denoting a distortion characteristic deteriorate in the conventional high-frequency amplifier. The reason of the deterioration of the characteristics will be described.
In the example shown in FIG. 16, the series resistor 6 having a resistance value of Rs is serially inserted between the output side of the amplifier 3 and the output matching circuit 5, and an output impedance of the amplifier 3 is equal to a value Zout. In this case, an output impedance having a value of Zout+Rs is obtained when the amplifier 3 is seen from the output matching circuit 5, and the output impedance Zout+Rs is changed to a characteristic impedance Zo in the output matching circuit 5. Therefore, an impedance Zout*+Rs obtained by seeing the output side of the conventional high-frequency amplifier from a stabilizing circuit composed of the series resistor 6 is equal to a value conjugated to the complex number Zout+Rs of the output impedance, and an output load impedance ZL obtained by seeing the output side of the conventional high-frequency amplifier from the amplifier 3 is equal to a value Zout*+2Rs conjugated to a complex number Zout+2Rs. Accordingly, the output load impedance ZL is higher than a conjugate complex impedance Zout* of the output impedance Zout of the amplifier 3 by 2Rs.
Also, in general, an output electric power of the amplified signal output from the amplifier 3 and the third intercept point are set to highest values in cases where the output load impedance ZL obtained by seeing the output side of a high-frequency amplifier from the amplifier 3 is slightly lower than the conjugate complex impedance Zout* of the output impedance Zout of the amplifier 3.
Therefore, because the output load impedance ZL obtained by seeing the output side of the conventional high-frequency amplifier from the amplifier 3 is higher than the conjugate complex impedance of the output impedance Zout of the amplifier 3 by 2Rs in the conventional high-frequency amplifier, in addition to the electric power loss occurred in the series resistor 6 which is inserted on the output side of the amplifier 3, a problem has arisen that the output electric power of the amplified signal output from the amplifier 3 and the third intercept point are lowered.
An object of the present invention is to provide, with due consideration to the drawbacks of the conventional high-frequency amplifier, a high-frequency amplifier and a high-frequency multistage amplifier in which an output signal having a high output electric power and a low distortion is obtained.
The object is achieved by the provision of a high-frequency amplifier comprising an amplifier for amplifying an input signal, and a stabilizing circuit, which is arranged on an output side of the amplifier and has a series resistor and a parallel resistor, for stabilizing the amplification of the input signal performed in the amplifier and outputting the amplified input signal.
In the above configuration, in cases where values of the series resistor and the parallel resistor are determined so as to set an output load impedance obtained by seeing the output side of the high-frequency amplifier from the amplifier to a value near to or slightly lower than a value of a conjugate complex impedance of an output impedance of the amplifier, an output signal having a high output electric power and a low distortion can be obtained in the high-frequency amplifier while keeping a gain and a noise characteristic of the high-frequency amplifier.
It is preferred that the stabilizing circuit is formed of a T-type circuit having a first series resistor, a second series resistor and the parallel resistor.
In this case, the gain of the high-frequency amplifier can be adjusted by adjusting the value of the first series resistor.
It is also preferred that the stabilizing circuit is formed of a xcfx80-type circuit having the series resistor, a first parallel resistor and a second parallel resistor.
In this case, the gain of the high-frequency amplifier can be adjusted by adjusting the value of the second parallel resistor.
It is also preferred that the stabilizing circuit is formed of a ladder-type circuit having the series resistor and the parallel resistor.
In this case, because the number of resistors in the ladder-type circuit is lower than that in the, T-type circuit or the xcfx80-type circuit, the configuration of the high-frequency amplifier can be simplified.
It is also preferred that the high-frequency amplifier further comprises a series inductor arranged between the amplifier and the stabilizing circuit, and a parallel condenser arranged between the amplifier and the stabilizing circuit.
Therefore, an output signal having a lower distortion can be obtained in the high-frequency amplifier.
The object is also achieved by the provision of a high-frequency multistage amplifier comprising a specific high-frequency amplifier, arranged in a first stage, for amplifying an input signal, and a general high-frequency amplifier, arranged in a stage following the first stage, for amplifying the input signal amplified by the specific high-frequency amplifier and outputting the amplified input signal as an output signal. The specific high frequency amplifier comprises an amplifier for amplifying the input signal, and a stabilizing circuit, which is arranged on an output side of the amplifier and has a series resistor and a parallel resistor, for stabilizing the amplification of the input signal performed in the amplifier.
In the above configuration, in cases where values of the series resistor and the parallel resistor of the stabilizing circuit are determined so as to set an output load impedance obtained by seeing the output side of the specific high-frequency amplifier from the amplifier to a value near to or slightly lower than a value of a conjugate complex impedance of an output impedance of the-amplifier, an output signal having a high output electric power and a low distortion can be obtained in the specific high-frequency amplifier while keeping a gain and a noise characteristic of the high-frequency amplifier. Therefore, an output signal having a high output electric power and a low distortion can be obtained in the whole high-frequency multistage amplifier while keeping a gain and a noise characteristic of the high-frequency amplifier.
Also, because the specific high-frequency amplifier arranged in the first stage of the high-frequency multistage amplifier has a high influence on the noise generated in the signal, the noise of the signal output from the high-frequency multistage amplifier can be efficiently lowered.
It is preferred that the stabilizing circuit of the specific high-frequency amplifier is formed of a T-type circuit having a first series resistor, a second series resistor and the parallel resistor.
In this case, the gain of the high-frequency amplifier can be adjusted by adjusting the value of the first series resistor.
It is also preferred that the stabilizing circuit of the specific high-frequency amplifier is formed of a xcfx80-type circuit having the series resistor, a first parallel resistor and a second parallel resistor.
In this case, the gain of the high-frequency amplifier can be adjusted by adjusting the value of the second parallel resistor.
It is also preferred that the stabilizing circuit of the specific high-frequency amplifier is formed of a ladder-type circuit having the series resistor and the parallel resistor.
In this case, because the number of resistors in the ladder-type circuit is lower than that in the T-type circuit or the xcfx80-type circuit, the configuration of the high-frequency amplifier can be simplified.